This invention relates to a focal point adjustment apparatus and method applied to microscopes used in the fields of medicine and biology.
Some of the factors that decrease the focusing accuracy are the effect of dirt adhering to subjects, the effect of dirt adhering to a plate member, such as a slide or a cover glass, on which a subject, such as biological tissue, is placed, and/or the effect of dirt adhering to a condenser lens that projects light on the plate member.
When deposits, such as dirt, adhere to the optical system, which is typically composed of a subject, a plate member and/or a condenser lens, the image of deposits gets mixed several thousand to several hundred .mu.m away from the image of the subject. The image of the deposits has a higher contrast than the image of the subject. In this case, when an automatic focusing operation is carried out, the microscope is focused on the deposits rather than the subject, causing the problem of lowering the focusing accuracy with respect to the subject.
The problem becomes more serious, depending on the type of plate member used. Specifically, for plate members, the standard slide has a thickness of 1.0 (-0.1 to +0.2) mm and the standard cover glass has a thickness of 0.17 (-0.02 to +0.01) mm. A plate member used for a microscope does not always use the standard slide or the standard cover glass. In this case, the above thicknesses are Japanese Industrial Standard. Depending on the wish of the observer and the nature of the subject, a slide and cover glass differing in thickness from the standard slide and cover glass, respectively, may be used.
The distance between the top surface position (reference in-focus position) of the slide put on the stage and the mounting position of the object lens to the revolver is generally fixed to a constant value. The constant value is usually a reference focal length of 45.0 mm. In the case of the standard slide of a thickness of 1.0 (.+-.0.2 to 0.3) mm, the distance between the stage top surface position and the mounting position of the object lens to the revolver is 46.0 (45.0+1.0) mm. The distance between the stage top surface position and the mounting position of the object lens to the revolver has a fixed value.
When the distance keep, and a slide and a cover glass that differ in thickness from the standard slide and the standard cover glass are used, however, the distance between the top surface position (reference in-focus position) of the slide and the mounting position of the object lens to the revolver has a value deviating from the reference focal length 45.0 mm. Therefore, when a slide and a cover glass that differ in thickness from the standard slide and the standard cover glass are used, the execution of automatic focusing may decrease the focusing accuracy with respect to the subject. Additionally, when deposits are on the plate member, such as a slide, as mentioned above, an optical system focuses to the deposits, and the focusing accuracy with respect to the subject decreases seriously.
One of the prior art techniques for solving such a problem is an area division distance-measuring system which adapts an area of without deposit to be elected, for use with video cameras or electronic still cameras.
With such techniques, however, control and image processing are too complicated and costly for application to microscopes.
An object of the present invention is to provide a practical focal point adjustment apparatus and method applied to a microscope which are capable of making a focus adjustment with high accuracy without being affected by the plate member or deposits, even when a plate member of a different thickness is used or when deposits are on the optical system.
Another object of the present invention is to provide a practical microscope capable of making a focus adjustment with high accuracy without being affected by the plate member or deposits, even when a plate member of a different thickness is used or when deposits are on the optical system.